I have mixed Maran hen/pullet about 20 weeks who I noticed was rather wobbly, went to pick her up and she is bone thin. She has 4 older sisters and they are doing just fine, going about their normal chicken business but Marie is sick. I was worried the older girls just weren't letting her eat and separated her yesterday she ate a little and drank, let her sleep inside over night. But now this afternoon she is worse, she is laying in the coop and really not moving, eyes closed.....no bugs or anything that I have noticed, she has always had a little cough since we got her. What is this?